Piezoelectric motor and method of exciting an ultrasonic traveling wave to drive the motor

ABSTRACT

A rotary ultrasonic piezoelectric motor is provided and a method of exciting a flexure traveling wave to drive the motor. The motor includes a stator having a piezoelectric ceramic disc polarized in the radial direction and bounded by a top electrode and a segmented bottom electrode. The motor also includes a power source for applying two pairs of alternating voltages to the bottom electrode segments to excite a shear-shear mode vibration in the stator, resulting in a shear-shear mode flexure traveling wave in the stator. The motor further includes a rotor operatively connected to the stator, and the stator is driven to rotate through a frictional force between the rotor and the stator due to the traveling wave deformation of the stator. A linear ultrasonic piezoelectric motor and method of exciting a flexure traveling wave to linearly drive the motor is provided. The motor includes a stator having a rectangular piezoelectric ceramic plate that is polarized in the longitudinal direction. The motor also includes a power source for applying two pairs of alternating voltages to the bottom electrode segments to excite a shear-shear mode vibration in the stator, resulting in a shear-shear mode flexure traveling wave in the stator. The motor further includes a slider operatively connected to the stator, and the stator is driven to move linearly through a frictional force between the slider and the stator due to the traveling wave deformation of the stator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a miniature piezoelectricmotor, and in particular to a rotary piezoelectric motor and a linearpiezoelectric motor and method of exciting an ultrasonic traveling wavefor driving the motor.

2. Description of the Related Art

Piezoelectric ultrasonic motors with their exceptional properties, suchas high resolution of displacement control, absence of parasiticmagnetic fields, frictional locking at the power-off stage, and highthrust to weight ratio, make them good candidates for use in precisionmicromechanical systems.

Piezoelectric motors have several advantages over a conventionalelectromagnetic motor. These include a faster response time, a highpower-to-weight ratio, and smaller packaging capability. They also haveseveral disadvantages, including the need for high voltage, highfrequency power sources, and potential wear at the rotor/statorinterface. These motors operate using a ferroelectric ceramic element toexcite ultrasonic vibrations in a stator structure. The ellipticalmovement of the stator is converted into the motion of a sliding platein frictional contact with the stator. The resulting movement is eitherrotational or linear, depending on the design of the structure.

Conventional piezoelectric ultrasonic motors can be mainly classifiedinto two classes: 1) traveling wave ultrasonic motors, and 2) standingwave ultrasonic motors. A conventional disc-type or ring-type travelingwave motor (rotary motor) is made from a piezoelectric disc (or ring)and a metal disc. With a piezoelectric d₃₁ effect, the piezoelectricdisc (or ring) and metal disc composite stator produces aflexure-flexure mode traveling wave that drives a contact rotor througha frictional force. The most frequently researched operating principlefor a linear piezoelectric motor is based on the excitation of alongitudinal and a superimposed bending mode of a rectangularpiezoelectric plate, to achieve the elliptic motion of the driving tip.

A disc-type traveling wave rotary motor, such as that developed byMatsushita Electric Company Ltd., is representative of a traveling wavetype of motor. The piezoelectric stator for this type of motor includesa composite disc comprised of a metal elastic disc and two piezoelectricceramic discs with thickness polarization. A traveling flexure vibrationmode is excited by each section of the piezoelectric discs, and producesa transverse width extension mode with d₃₁ effect under two ac voltages.

Another typical configuration for the conventional traveling waveultrasonic rotary piezoelectric motor is a ring-type ultrasonic motor,such as that developed by Sashida. This motor utilizes the axialbending-vibration mode of a circular ring with d₃₁ effect of thepiezoelectric ceramic ring. The piezoelectric ring-type element ispolarized in the thickness direction and produces a transverse lengthextension mode with d₃₁ effect under ac voltages.

A typical configuration of the conventional linear ultrasonic motor,such as that developed by Yoshiro Tomikawa, is driven by the ellipticmotion of the combination displacement of longitudinal (d₃₁) andsecondary bending modes. The motor operates according to the principlethat at a certain distance to length ratio of a rectangular-shapedpiezoelectric ceramic plate, the resonant frequencies of firstlongitudinal and second bending modes coincide with each other. Theelliptic motion is obtained by the combination of the two vibrations.

While all the above described rotary and linear motors offersatisfactory performance, they primarily utilize a transverse lengthextension mode for exciting a traveling flexure wave with a low d₃₁piezoelectric effect and low k₃₁ electromechanical coupling effect. Inthe operational mode, the relatively low d₃₁ and k₃₁ effects of thepiezoelectric ceramic material hinders additional development of thesetypes of motors.

Thus, there is a need in the art for a system and method of providing arotary motor which utilizes a rotary shear vibration mode and a linearultrasonic motor which uses the linear shear vibration mode,respectively, of piezoelectric ceramics.

SUMMARY OF THE INVENTION

Accordingly, the present invention is a piezoelectric motor and a methodof exciting an ultrasonic traveling wave for driving the motor. A rotaryshear motor includes a stator having a piezoelectric ceramic discpolarized in the radial direction and bounded by a top electrode and abottom electrode divided into segments. The rotary motor also includes apower source for applying two pairs of alternating voltages to thebottom electrode segments to excite a shear-shear mode vibration in thestator, resulting in a shear-shear mode flexure traveling wave in thestator. The rotary motor further includes a rotor operatively connectedto the stator, and the portion of the rotor in contact with the statoris driven to rotate through a frictional force between the rotor and thestator due to the traveling wave deformation of the stator.

A linear ultrasonic piezoelectric motor includes a stator having arectangular piezoelectric ceramic plate that is polarized in thelongitudinal direction. The linear motor also includes a power sourcefor applying two pairs of alternating voltages to the bottom electrodesegments to excite a shear-shear mode vibration in the stator, resultingin a shear-shear mode flexure traveling wave in the stator along thelongitudinal direction of the stator. The linear motor further includesa slider operatively connected to the stator, and the portion of theslider in contact with the stator is driven to move linearly through africtional force between the slider and the stator due to the travelingwave deformation of the stator.

The method of exciting a shear-shear mode vibration in a piezoelectricceramic disc for a rotary ultrasonic piezoelectric motor includes thesteps of polarizing the piezoelectric disc in a radial direction,applying a pair of alternating voltages with a phase shift of 90 degreesto the bottom electrode segments from a power source, exciting ashear-shear mode vibration in the stator, and producing a shear-shearmode flexure traveling wave in the stator causing a portion of the rotorin contact with the stator to rotate through a frictional force betweenthe rotor and the stator due to the traveling wave deformation of thestator.

The method of exciting a shear-shear mode vibration in a piezoelectricceramic plate for a linear ultrasonic piezoelectric motor includes thesteps of polarizing the piezoelectric plate in a longitudinal direction,applying a pair of alternating voltages with a phase shift of 90 degreesto the bottom electrode segments from a power source, exciting ashear-shear mode vibration in the stator, and producing a shear-shearmode flexure traveling wave in the stator causing a portion of theslider in contact with the stator to move linearly through a frictionalforce between the slider and the stator due to the traveling wavedeformation of the stator.

One advantage of the present invention is that a piezoelectric motor andmethod of exciting an ultrasonic traveling wave for driving the motor isprovided. Another advantage of the present invention is that a systemand method of exciting an ultrasonic traveling wave using a shear modefor driving the motor is provided that is reduced in size, but operateswith increased efficiency because d₁₅ and k₁₅ effects are larger thand₃₁ and k₃₁ effects, respectively, in the PZT piezoelectrics. Stillanother advantage of the present invention is that a piezoelectric motorand method of exciting an ultrasonic traveling wave using a shear modefor driving the motor is provided that realizes increased mechanicalenergy output due to a higher k₁₅ effect. A further advantage of thepresent invention is that the electrical to mechanical conversion rateis increased, so that the motor can be miniaturized. Still a furtheradvantage of the present invention is that the piezoelectric ceramicdisc is polarized using a shear-shear flexure motion mode to produce arotary motion of the rotor. Still yet a further advantage of the presentinvention is that the piezoelectric ceramic rectangular plate ispolarized using a linear shear-shear flexure motion mode to produce alinear motion of the slider.

Other features and advantages of the present invention will be readilyappreciated, as the same becomes better understood after reading thesubsequent description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a bottom view of a piezoelectric ceramic disc with radialpolarization direction for a rotary piezoelectric motor, according tothe present invention.

FIG. 1 b is a cross-sectional view taken along line a-a through thepiezoelectric ceramic disc of FIG. 1 a, according to the presentinvention.

FIG. 2 is a table illustrating the properties of a hard-typepiezoelectric ceramic material for the motor of FIG. 1 a, according tothe present invention.

FIG. 3 is a table illustrating an example of a resonance frequencyconstant for the motor of FIG. 1 a, according to the present invention.

FIG. 4 is a perspective view of a piezoelectric ceramic disc with a B₀₂mode traveling flexure wave produced by the shear-shear vibration modeof the piezoelectric ceramic disc under two phase voltages and freeboundary condition with phase difference of π/2, according to thepresent invention.

FIG. 5 is another example of a perspective view of a piezoelectricceramic disc with a B₁₂ mode traveling flexure wave produced by theshear-shear vibration mode of the piezoelectric ceramic disc under twophase voltages and free boundary condition with phase difference of π/2,according to the present invention.

FIG. 6 is a cross-sectional view of the shear-shear B₀₂ modepiezoelectric motor, according to the present invention.

FIG. 7 is another example of a cross-sectional view of the shear-shearB₁₂ mode piezoelectric motor, according to the present invention.

FIG. 8 is a table illustrating an example of a resonance frequencyconstant for the motor of FIG. 9(a), according to the present invention.

FIG. 9(a) illustrates another embodiment of a linear shear-shear modepiezoelectric linear motor, according to the present invention.

FIG. 9(b) is a top view illustrating the tooth structure for theshear-shear mode piezoelectric linear stator of FIG. 9(a), according tothe present invention.

FIG. 10 is a perspective view illustrating a rectangular piezoelectricceramic plate for the motor of FIG. 9(a), according to the presentinvention.

FIG. 11 is a flowchart illustrating a method of exciting a flexuretraveling wave using a shear-shear mode to drive the rotor in a rotarydirection, according to the present invention.

FIG. 12 is a flowchart illustrating a method of exciting a flexuretraveling wave using a linear shear-shear mode to drive the slider in alinear direction, according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIGS. 1-7 a shear-shear piezoelectric motor 10 isillustrated. The piezoelectric ceramic disc described in the presentinvention is especially advantageous in enhancing the performance of anultrasonic piezoelectric ceramic motor. In particular, the vibrationmode selected for the piezoelectric stator has a piezoelectric effectand electromechanical coupling effect that is higher than those used inconventional rotary or linear ultrasonic motors using a comparablepiezoelectric ceramic material.

Referring to FIGS. 1 a and 1 b, the shear-shear piezeoelectric motor 10includes a traveling wave composite stator 12. The stator 12 includes abody portion (not shown) for housing a piezoelectric ceramic disc 16,which in this embodiment is polarized in the radial direction, as shownat 30. It should be appreciated that the piezoelectric disc 16 for theshear-shear motor 10 may be made from a hard-type piezoelectric ceramic.A typical piezoelectric ceramic material for such high powerapplications is a hard-type material, such as PZT8. An example of such amaterial is APC841 from APC Company.

FIG. 2 is a table listing parameters for the ceramic disc 16, such asthe electromechanical coupling coefficients, piezoelectric constants andother parameters for a hard-type piezoelectric ceramic material, asshown at 28. For example, a piezoelectric ceramic disc 16 may haveproperties of high d₁₅ performance, piezoelectric coefficient, highelectromechanical coupling coefficient and high quality factor. In thisexample, a piezoelectric disc-type stator 12 having an outer diameter of10.5 mm, and inner diameter of 1.8 mm and thickness of 0.5 mm, wasoperated in the B₀₂ mode with the resonance frequency of about 39 kHz.Under 70 Vrms/10 mA, the motor speed was about 200 rpm with the maximumtorque around 1.8 mNm. It should be appreciated that the torque obtainedwith this motor 10 is larger than 0.7 mNm torque obtained from aconventional unimorph-type motor with a similar configuration.

The piezoelectric disc 16 is bounded by at least one metal plate 18 inorder to excite a flexure mode of the composite stator 12. The plate 18includes a top electrode 20 and a bottom electrode 22. The bottomelectrode 22 is divided into four segments 24, labeled a, b, c and d, asshown in FIG. 1 b. Two pairs of alternative voltages 26, V₀ sin ωt andV₀ cos ωt, are each applied to parts a, c and b, d, respectively. Thenumber of electrode segments can be chosen any even number (2, 4, 6, 8,. . . ). Preferably the top electrode 20 of the ceramic disc 16 is notdivided into parts, in order to provide a ground.

As shown in FIG. 2 at 28, the piezoelectric ceramic material's d₁₅ andk₁₅ effects are about 4 times and 2 times higher than the correspondingd₃₁ and k₃₁ effects. It should be appreciated that by using the d₁₅ andk₁₅ effect from a shear mode to excite a flexure mode or traveling wavein a disc or ring-type piezoelectric stator, an ultrasonic motor withhigher electromechanical coupling effect can be obtained. It should alsobe appreciated that although the piezoelectric constant d₃₃ and couplingcoefficient k₃₃ also are higher than the piezoelectric constant d₃₁ andcoupling effect k₃₁, it is difficult to excite a flexure traveling waveusing the d₃₃ effect for a disc or rectangular type miniaturized motor.

Referring back to FIG. 1 b, the shear-shear mode is illustrated for thepiezoelectric ceramic disc shown in FIG. 1 a. Since the applied electricfield is vertical to the polarization direction as shown at 30, theapplied voltages 26 V₀ sin ωt and V₀ cos ωt will induce thepiezoelectric disc to produce a shear mode vibration of the stator. As aresult of the shear mode concurrent vibration and rotation, ashear-shear flexure traveling wave is excited on the stator 12, as shownat 31. It should be appreciated that a method of exciting a flexuretraveling wave in the proposed motor (to be described) isdistinguishable from that of a conventional piezoelectric disc/ringhaving a metal elastic plate composite rotor. The present inventionadvantageously utilizes two shear strain modes (d₁₅ effect) with reversestrain direction of the piezoelectric ceramic for exciting a flexuretraveling wave on the stator 12. This is distinguishable from a priorart piezoelectric disc/ring composite stator which utilizes a transversewidth or length extension mode of the piezoelectric disc under anapplied ac voltage.

The piezoelectric ceramic disc stator has a flexure vibration moderelated to its operational frequency. For example, if the elasticcomposite disc 16 is operated at a bending mode B_(m,n), its resonancefrequency at the free-free boundary condition can be given by theequation: $\begin{matrix}{f_{r} = {\frac{1}{2\pi}\frac{\alpha_{mn}^{2}}{a^{2}}h\sqrt{\frac{E}{3{\rho\left( {1 - \sigma^{2}} \right)}}}}} & (1)\end{matrix}$

where α_(mn) is a resonance frequency constant and a, 2h are the radiusand thickness of the disc vibrator, respectively; E, ρ and σ are Young'smodulus, mass density and Poisson ratio, respectively.

FIG. 3 is a table illustrating an example of a resonance frequencyconstant α_(mn) for different flexure resonance modes B_(mn), as shownat 32. It should be appreciated that these types of frequency constantscan be utilized to calculate the resonance frequency for each mode.

Referring to FIG. 4, an example of a ceramic disc 16 with travelingflexure wave as shown at 34 is illustrated for the B₀₂ mode produced byshear-shear mode of the piezoelectric ceramic disc under two phasevoltages with phase difference of π/2. It should be appreciated that theceramic disc with the traveling flexure wave may be modeled using ananalytical technique, such as a finite element analysis (FEA). Forexample, an FEA program, such as Analysis of Transducers by Integrationof LaPlace Equations (ATILA) may be used to predict the operation of thestator 12 structure. Using FEA, ATILA provides both the resonancefrequencies of the stator structure and simulations of its motion modes.Moreover, using FEA modeling, some useful rotational modes for exitingflexural traveling modes may be discovered.

Referring to FIG. 5, another example of a ceramic disc 16 with thetraveling flexure wave is illustrated for the B₁₂ mode produced by theshear-shear mode of the piezoelectric ceramic disc under two phasevoltages with a phase difference of π/2. This traveling flexure wave mayalso be modeled using the FEA. The mode rotation in the B₀₂ and B₁₂modes may follow the sequences: a-b-c . . . -I-a.

Referring to FIG. 6, a piezoelectric shear-shear mode motor using theB₀₂ mode produced by the shear-shear mode of the piezoelectric discunder two phase voltages with a phase difference of π/2 is illustratedat 38. It should be appreciated that like features have like referencenumbers. The motor 38 includes a piezoelectric ceramic disc-type stator12, as previously described. The stator 12 includes a ring containing aplurality of teeth 40 bonded onto the piezoelectric disc 16. Preferably,the teeth ring 40 is a thin metal material, such as stainless steel orthe like. The teeth ring 40 serves as an amplifier for amplifying theflexure vibration of the piezoelectric disc 16.

The shear-shear motor 38 also includes a rotor 42 operatively positionedwith respect to the stator 12, and a pressing mechanism 44 forpositioning the rotor with respect to the stator. In this example, thepressing mechanism is a lever arm 54 and ball 56. The motor 38 alsoincludes a shaft 46 extending through a centrally located aperture 48,50 in each the rotor 42 and the stator 12. The shaft 46 is secured tothe piezoelectric ceramic disc stator 12 using an adhesive, such asepoxy resin or the like. The stator 12 is preferably secured to themotor by a holding means 58, as is known in the art.

The piezoelectric disc 16 includes a top electrode 20 as previouslydescribed, which serves as a common electrode, and a bottom electrode 22which is divided into four segments 24 for two pairs of applied acvoltage input 26. The rotor 42 is held elastically in contact with theteeth ring 40 for the rotational driving thereof by the traveling waveproduced on the stator 12. For example, the lever arm 54 and ball 56 maybe utilized to pre-load the rotor 42 against the stator 12.

When a traveling flexure wave is excited along the circumferencedirection of the disc, the portion of the rotor 42 that is in contactwith the stator 12 is driven to rotate through the frictional forcebetween the rotor 42 and the stator 12. By changing the phase differenceof two inputted voltage pairs from 90 degrees to −90 degrees, therotational direction of the motor changes in a corresponding manner.

Referring to FIG. 7 another example of the shear-shear mode motor usingB₁₂ mode is illustrated 60. It should be appreciated that likecomponents have like reference numbers. The motor 60 includes apiezoelectric ceramic disc-type stator 12, as previously described. Themotor 60 also includes a rotor 42 and a pressing mechanism 44. Thismotor is distinguishable from the B₀₂ mode motor, since the diameter ofthe metal teeth ring 40 is smaller. The teeth ring is placed on thepiezoelectric disc 16 to induce a wave peak for the B₁₂ vibration mode.The teeth ring 40 also serves as an amplifier for amplifying flexurevibration of the piezoelectric disc 16. A shaft 46 is fixed in thecenter of the piezoelectric ceramic disc stator 12 using an adhesivesuch as an epoxy resin, or the like. In order to obtain a more compactstructure, the rotor 42 may be pre-loaded against the stator 12 using asmall coil spring 52. The top electrode 20 of the piezoelectric disc 16is common electrode, and the bottom electrode 22 is divided into foursegments for two pairs of ac voltage input, as previously described.

In operation, when a traveling flexure wave is excited along thecircumference direction of the disc 16, the portion of the rotor 42 thatis in contact with the stator 12 is driven to rotate through thefriction force between the rotor 42 and the stator 12. By changing thephase difference of the two input voltage pairs from 90 degrees to −90degrees, the rotational direction of the motor 60 may be changed.

It should be appreciated that other work modes, such as B₀₃, B₀₄, B₀₅,B₀₆ . . . B₁₃, B₁₄, B₁₅, B₁₆ . . . , are also possible for theshear-shear mode traveling wave motor. These other work modes mayrequire a corresponding redesign of the electrode configuration of thepiezoelectric ceramic disc 16.

In an alterative embodiment shown in FIGS. 8-10, the motor is ashear-shear mode linear motor 70. In this example, the stator 72includes a rectangular piezoelectric ceramic plate 74 having apolarization in the longitudinal direction as shown at 76. It should beappreciated that the ceramic plate may have an alternative polarizationas shown in FIG. 10 at 76 a. The motor also includes a rectangular metalplate or slider 86 operatively connected to the stator 72. If analternating current electrical field is applied in the thicknessdirection as shown at 80, the thickness shear mode vibration will beexcited. A shear mode vibration coupled with an orthogonal mode isexcited and a linear motion is obtained in the longitudinal direction bythe resulting traveling wave produced by the piezoelectric ceramics.Experimental results demonstrate that the points on the surface of theplate generate elliptic motions. The linear piezoelectric motor 70 isthus obtained using the thickness shear mode vibration.

Referring to Table 8, an example of different resonance frequencies andvibrations for the piezoelectric plate are illustrated at 82. Ananalytical technique, such as FEA, may be utilized to determine theresonant frequencies of the shear-linear mode piezoelectric vibration.In this example, the dimensions of the piezoelectric plate 74 areapproximately 12 mm in length, 5 mm in width and 1 mm in thickness. Thevibration mode at 507.07 kHz corresponds to the thickness shear mode(which is a shear mode coupled with an orthogonal thickness vibrationmode). It is contemplated that the resonance frequency of the thicknessshear mode is dependent on the ratio of length and thickness of theplate, and is independent of width.

Referring back to FIG. 9(a), the shear linear mode motor 70 includesonly one piezoelectric ceramic plate bounded by a first electrode 88 anda second electrode 90. Preferably, the second electrode 90 is dividedinto segments, as shown at 92. The first electrode serves as a ground.The motor 70 also includes a tooth-like plate 84, similar to thepreviously described tooth ring disposed between the stator 72 and theslider 86. The tooth plate 84 is made of a metal material, such asbrass. The piezoelectric materials are hard piezoelectric ceramics, suchas that manufactured and sold under the name APC841 from APC Company, aspreviously described. The rectangular piezoelectric plate 74 ispolarized in the longitudinal direction 76.

Referring to FIG. 9(b), a top view of the tooth plate 84 structure isillustrated in more detail. The tooth plate 84 is utilized to amplifythe displacement of the wave on the surface of the ceramic plate 74. Theapplication of the applied current causes the motor to easily move therectangular plate 74 in a forward direction.

Referring to FIG. 11, a method of exciting a shear-shear mode vibrationin the piezoelectric ceramic disc 16 for the previously described rotaryultrasonic motor 10 is illustrated. The method begins in block 100 withthe step of polarizing the piezoelectric disc in a radial direction. Anexample of a disc polarized in the radial direction is shown in FIGS. 1b, 6 and 7.

The method advances to step 110, and an electric field 26, such as avoltage, is applied to the piezoelectric disc 16, and in particular thebottom electrode, in a thickness direction. The voltage is preferablyapplied as a pair of alternating voltages with a phase shift of 90degrees to the segments, as previously described. An example of avoltage is 50 volts. The method advances to block 120.

In block 120, a shear rotary vibration motion is excited on the disc, asa result of the applied voltage. It should be appreciated that the useof the piezoelectric ceramic material's piezoelectric effect andelectromechanical coupling effect is maximized through the selection ofthe shear vibration mode. For example, the d₁₅ and k₁₅ effects of shearmode in piezoelectric ceramic materials are about three times that ofthe d₃₁ and two times of k₃₁ effects, respectively. FIG. 2 shows acomparison of the piezoelectric constants and electromechanical couplingconstants.

The method advances to block 130.

In block 130, a flexure vibration shear-shear traveling wave producedalong the circumferential direction of the disc results in a deformationof the surface of the stator 12. The portion of the rotor in contactwith the stator is driven to rotate through the frictional force betweenthe rotor and the stator. The shear rotary motor advantageously hascharacteristics of lower driving current, thus, higher energy convertingefficiency and higher generative torque than those in the conventionalflexure k₃₁ type motor with a similar size/configuration.

Referring to FIG. 12, a method of exciting a shear-shear mode vibrationin the piezoelectric ceramic plate 74 for the linear ultrasonic motor70, as previously described with respect to FIGS. 8-10. The methodbegins in block 150 with the step of polarizing the piezoelectric plate74 in a longitudinal direction 76, or an alternatively longitudinaldirection 76 a. An example of a ceramic plate 74 polarized in thelongitudinal direction is shown in FIGS. 9 a, and 10.

The method advances to step 160, and an electric field 78, such as avoltage, is applied to the piezoelectric rectangular plate 74, and inparticular the bottom electrode, in a thickness direction. The voltageis preferably applied as a pair of alternating voltages with a phaseshift of 90 degrees to the segments, as previously described. An exampleof a voltage is 55 Vrms. The tooth plate 84 advantageously amplifies theapplied voltage. The method advances to block 170.

In block 170, a linear shear-shear vibration motion is excited on thepiezoelectric ceramic plate 74, as a result of the applied voltage. Themethod advances to block 180.

In block 180, a flexure vibration shear-shear traveling wave producedalong the length direction of the rectangular plate results in adeformation of the surface of the stator 12. The rectangular plate 74moves in a linear direction, resulting in the corresponding motion ofthe slider for the linear motor 70.

The present invention has been described in an illustrative manner. Itis to be understood that the terminology which has been used is intendedto be in the nature of words of description rather than of limitation.

Many modifications and variations of the present invention are possiblein light of the above teachings. Therefore, within the scope of theappended claims, the present invention may be practiced other than asspecifically described.

1-10. (canceled)
 11. A linear ultrasonic piezoelectric motor comprising:a stator, wherein said stator includes a piezoelectric ceramic platethat is rectangular and polarized in the longitudinal direction andbounded by a top electrode and a bottom electrode divided into segments;a power source, wherein said power source applies two pairs ofalternating voltages to said bottom electrode segments to excite a shearmode vibration in said stator, resulting in a shear-shear mode flexuretraveling wave in said stator; and a slider operatively connected tosaid stator, wherein said portion of said slider in contact with saidstator is driven to move linearly through a frictional force betweensaid slider and said stator due to said traveling wave deformation ofsaid stator.
 12. The linear motor as set forth in claim 11 furthercomprising a metal plate having a plurality of teeth disposed on saidceramic disc.
 13. The linear motor as set forth in claim 11 wherein saidbottom electrode is divided into an even number of segments.
 14. Thelinear motor as set forth in claim 11 wherein said stator and saidslider are each supported on a shaft extending through a centrallylocated aperture in said slider and a centrally located aperture in saidstator.
 15. The linear motor as set forth in claim 11 further comprisinga holding means for supporting said stator.
 16. The linear motor as setforth in claim 11 further comprising a pressing means for preloadingsaid stator against said slider.
 17. The linear motor as set forth inclaim 16 wherein said pressing means is a spring.
 18. The linear motoras set forth in claim 11 wherein said piezoelectric plate is operated ata flexure resonance mode of B₀₂.
 19. The linear motor as set forth inclaim 11 wherein said piezoelectric plate is operated at a flexureresonance mode of B₁₂.
 20. The linear motor as set forth in claim 1wherein said phase difference of said applied voltage is changed from 90degrees to −90 degrees to change the linear motion of said slider. 21.(canceled)
 22. (canceled)
 23. A method of exciting a shear-shear modevibration in a piezoelectric ceramic plate for a linear ultrasonicpiezoelectric motor having a slider and a stator, said method comprisingthe steps of: polarizing the piezoelectric plate in a longitudinaldirection, wherein the piezoelectric plate is bounded by a topelectrode, and a bottom electrode divided into segments; applying a pairof alternating voltages with a phase shift of 90 degrees to the bottomelectrode segments from a power source; exciting a shear-shear modevibration in the stator; and producing a shear-shear mode flexuretraveling wave in the stator causing a portion of the slider in contactwith the stator to move linearly through a frictional force between theslider and the stator due to the traveling wave deformation of thestator.
 24. A method as set forth in claim 23 further comprising thestep of changing the phase difference of the applied voltage from 90degrees to −90 degrees to change the linear direction of the slider. 25.A method as set forth in claim 23 further including the step ofamplifying the flexure vibration of the plate using a plate having aplurality of teeth disposed on the piezoelectric plate.